1. Field of the Invention
The present invention relates generally to patient fluid monitoring systems and more specifically to monitoring systems using flame photometry.
2. Discussion of the Prior Art
Human patients undergoing surgery lose fluids which require replacement at rates depending upon the location and type of surgery. Peripheral procedures require the least fluids while intra-abdominal procedures require the most. Some procedures even introduce more fluid into patients than is removed. Procedures such as transurethral bladder and prostate resections, and hysteroscopic vaginal hysterectomies using large volumes of irrigating solutions may result in absorption of significant amounts of the irrigating solution accompanied by increased intravascular volume and the dangers of congestive heart failure and hyponatremia (see J. C. Ayus & A. I. Arieff, Glycine-induced Hypoosmolar Hyponatremia, 157 Arch. Intern. Med. 223 (1997), which is hereby incorporated by reference).
Much of the rationale for administering large quantities of postoperative fluids, despite the attendant dangers of pulmonary edema, includes the effects of the "third space," regions in the body occupied by fluids which are not in equilibrium with the bloodstream. Examples of the third space include bums, bruises, traumatized operative bed (intra-abdominal or intra-thoracic), traumatically injured tissues, and infarcted tissues. Fluids in the third space are literally out of circulation and therefore hemodynamically inactive. Fluid sequestration in the third space is a circulation and therefore hemodynamically inactive. Fluid sequestration in the third space is a unique kind of fluid loss in postoperative patients (see M. H. Rosenthal & A. I. Arieff, Fluid and Electrolyte Therapy in Critically Ill Patients and Those Who Are Pre-, Post-, or Intraoperative, in Fluid, Electrolyte and Acid-Base Disorders 597 (A. I. Arieff & R. A. DeFronzo eds., 1995), which is hereby incorporated by reference).
However, there is no simple bedside method for accurately measuring these fluid losses, and in actual practice clinical approximation determines replacement. The sequestered extracellular fluid (third space losses) postoperatively during an uncomplicated procedure varies between negligible and 3 liters. Quantification of functional extracellular fluid using the available means of measuring is extremely difficult, and consequently no accurate a priori formula for intraoperative fluid administration has been derived.
Postoperative fluid balance depends on underlying pathology factors including anesthesia, intraoperative fluid therapy and intra- and post-operative complications. Humoral mediators (such as the renin-angiotensin system, catecholamines, aldosterone, and AVP), which can influence hemodynamics and are released during surgery as described below, may persist into the postoperative period and require continuous administration of large volumes of fluids. While the intravascular volume must be maintained to avoid postoperative renal insufficiency, too much postoperative fluid can result in heart and lung failure with pulmonary edema. The potential postoperative complication of pulmonary edema and respiratory failure is a major hazard which discourages administering fluids in sufficient quantities to maintain preload.
The quantity of fluid necessary to induce pulmonary edema varies according to individual patient factors such as age, body weight, tissue turgor, cardiac function, pulmonary function, renal function, plasma vasopressin levels, and plasma proteins. The literature includes some information concerning minimal quantities of fluid which could induce pulmonary edema in otherwise generally healthy postoperative patients, but this information does not imply that any given quantity of fluid will necessarily induce pulmonary edema. Little information is available concerning the maximum postoperative volume of fluid which can be safely administered. In particular, it is not clear what volume of fluid might result in pulmonary edema in a postoperative patient who does not have serious cardiovascular, hepatic or renal disorders.
As shown in FIG. 1, three major postoperative complications, hypernatremia, hyponatremia, and pulmonary edema affect almost 650,000 postoperative patients, with an estimated mortality of 78,000 individuals, per year in the USA (see M. H. Rosenthal & A. I. Arieff, Fluid and Electrolyte Therapy in Critically Ill Patients and Those Who Are Pre-, Post-, or Intraoperative, in Fluid, Electrolyte and Acid-Base Disorders 597-632 (A. I. Arieff & R. A. DeFronzo eds., Churchill Livingstone, N.Y., 1995).
The art related to the field of systematic monitoring of the fluid and electrolyte balances in patients includes partial solutions to the above-described problems. There is no prior art comprehending a systematic approach which can warn physicians when a major problem (hyponatremia, hypernatremia, pulmonary edema) is imminent, and give meaningful suggestions to an attending physician. In many hospitals' operating rooms the fluid input and output volumes are roughly estimated by an attending physician aided only by his or her visual observations and experience.
Among prior inventions directed towards certain aspects of the fluid and electrolyte balance problem, Parrish (U.S. Pat. No. 4,448,207) and Blankenship, et al. (U.S. Pat. No. 4,658,834) both disclose apparatuses using sonic transducers for measuring of the volume of fluids outgoing from a patient. Corbitt, et al. (U.S. Pat. No. 4,449,538) discloses an apparatus which measures bulk fluid input and output volumes and advises an attending physician, but not on electrolyte balance. Cormier, et al. (U.S. Pat. No. 4,531,088) discloses in-line blood analysis through electrical resistance measuring, and Oppenheimer (U.S. Pat. No. 5,331,958) does the same through laser beams. Micklish (U.S. Pat. No. 5,285,682) addresses the problem of measuring the volume of fluid absorbed in sponges. Ludwigsen (U.S. Pat. No. 5,236,664) addresses the problem of losing blood in non-fluid forms by measuring levels of hemoglobin in blood-containing materials to estimate total blood loss.
Measurement of the concentration of electrolytes, in particular sodium and potassium, in body fluids such as blood or urine has been aimed almost exclusively at getting high accuracy readings about some nominal normal body level. There are several ways of doing this, for example using a device called a flame photometer in which the thermal energy of a flame ionizes some atoms from a sample, emissions from which color the flame to obtain a quantitative estimate of the properties of the fluid being analyzed. The most often used emission lines for sodium and potassium are sodium at 589 nm and potassium 767 nm. The prior art also describes the use of sparks, including sparks created by lasers and by either DC or AC electrical discharges. The prior art generates a spark by using two electrodes immersed in the fluid. This poses problems associated with unknown effects that the optical properties of the fluid might have. For example, if two fluids contained the same electrolyte concentrations but one was a different color than the other, the resulting light emitted and available for analysis might be affected. In addition, the fluid composition is highly variable, may be full of debris, and is potentially infectious.
For purposes of patient fluid monitoring it is important to know or measure electrolyte concentrations in input and output fluids, which can vary significantly, although accuracy within twenty percent is probably good enough. It is also desirable that the sensor be disposable and inexpensive.